1. Field of the Invention
The present invention relates to a tunable wavelength-selective filter capable of tunably selecting any optical signal from among wavelength-division multiplexed optical signals in an optical fiber, and to a manufacturing method for fabricating a high performance, inexpensive tunable wavelength-selective filter.
2. Description of the Related Art
Optical communication systems employing optical fibers have been rapidly put into practice because they can transmit large capacity information at a high speed. At present, however, they can only transmit only optical pulses with a particular wavelength. If they can transmit optical pulses at multiple different frequencies, they will be able to transmit larger capacity of information. This technique is called wavelength-division multiplexing (WDM), and is intensively researched. To achieve the wavelength-division multiplexing, a tunable wavelength-selective filter is necessary which can select only an optical signal of a desired wavelength from among optical pulses with multiple wavelengths.
As conventional tunable wavelength-selective filters of such a kind, there are a grating filter which controls its angle with a motor, a tunable wavelength-selective filter which moves its dielectric filter with a motor, and an etalon filter which controls its cavity length with a piezoelectric element. However, since they are mechanically controlled, they have drawbacks of being slow in response, bulk in module size, and expensive in cost.
To solve the foregoing problems of the tunable wavelength-selective filter, the assignee of the present application proposed a filter capable of varying the optical gap of a Fabry-Perot etalon by applying a voltage across a nematic liquid crystal material filled in the etalon as disclosed in Japanese Patent Application Laying-open No. 220,618/1992. Since the tunable wavelength-selective filter is a planar type device, it has an advantage of being able to be arrayed easily.
The filter using the nematic liquid crystal material, however, has a problem in that it cannot achieve a tuning rate beyond the maximum response rate of the nematic liquid crystal material of about a few milliseconds. A high tuning rate is an essential characteristic for implementing the wavelength-division multiplexing (WDM) in a high speed LAN (local-area network). In addition, since the liquid crystal filters utilize the orientations of the liquid crystal materials varied by applied voltages, their variations in refractive index are not uniform depending on the polarization state or direction of a plane of polarization of incident light. This causes so-called polarization dependence in which the tuning characteristics vary in response to the plane of polarization, which presents a problem in practical use. It is reported that a tunable wavelength-selective filter employing a chiral smectic A has a high response rate of about 10 microseconds. However, it has only a narrow tunable range, and its polarization dependence has yet to be improved (A. Sneh and K. Johnson, xe2x80x9cHigh-speed continuously tunable liquid crystal filter for WDM networkxe2x80x9d, J. Lightwave Technol. vol. 14, p. 1067, 1996). Accordingly, to implement the WDM in high speed optical LANS, it is desired to develop tunable wavelength-selective filters of being able to solve the foregoing two problems, that is, fast response, polarization-independent tunable wavelength-selective filters of highly practical utility.
On the other hand, low cost, highly reliable, fixed wavelength filters with optical fibers have been developed (T. Oguchi, J. Noda, H. Hanafusa and S. Nishi, xe2x80x9cDielectric multilayered interference filters deposited on polyimide filmsxe2x80x9d, Electronics Letters, Vol. 27, pp. 706-707, 1991).
The fixed wavelength filters with optical fibers are each fabricated by forming a dielectric mirror film on a polyimide film, by dicing it into few millimeter squares, and by embedding them into slots formed in the optical fibers. The fixed wavelength filters with optical fibers employ the polyimide film instead of glass as a substrate, which makes it possible for the dielectric mirror to be diced into few millimeter squares, and to be thinned to about a few tens of micrometers of thickness, enabling them to be inserted into the slots formed in the optical fibers. This obviates optical components such as lenses, and tedious alignment, making it possible to implement a very cheap and highly reliable fixed wavelength filters. However, it is difficult for the fixed wavelength filters with optical fibers to tunably control the filter transmission wavelength.
The present invention is implemented to solve the foregoing problems involved in the conventional techniques. It is therefore an object of the present invention to provide a polarization-independent tunable wavelength-selective filter with a high response rate and high speed tuning capability by utilizing as a cavity a material in which liquid crystal droplets are dispersed in a light transmissive characteristic medium.
Another object of the present invention is to provide a tunable wavelength-selective filter capable of increasing its tunable range per voltage by adding plasticizer into a polymer material in which the liquid crystal droplets are dispersed and by thus reducing the driving voltage of the liquid crystal droplets, although the liquid crystal droplets have an inclination to increase the driving voltage as their sizes decrease.
Still another object of the present invention is to provide a tunable wavelength-selective filter which is lower in cost and higher in reliability than the conventional tunable wavelength-selective filters, and capable of implementing higher density and smaller size by combining with arrayed optical fibers.
Another object of the present invention is to provide a tunable wavelength-selective filter capable of improving its spectrum characteristics, and of extending its wavelength tunable range to the entire wavelength range by stacking multiple tunable wavelength-selective filters or by using a planar mirror or prism mirror.
A still another object of the present invention is to provide a tunable wavelength-selective filter capable of simply constituting an optical multiplexer or demultiplexer by using a planar mirror.
In order to solve the problems aforementioned, a tunable wavelength-selective filter of a Fabry-Perot etalon type according to the present invention is constructed such that it comprises: a first layer including a transparent electrode and an optical mirror layer; a third layer including a transparent electrode and an optical mirror layer; and a second layer which is composed of a material with a refractive index variable with electric field, and sandwiched between the first layer and the third layer, wherein the material having a refractive index variable with electric field is composed by dispersing liquid crystal droplets equal to or less than 150 nm in diameter in a light transmissive characteristic medium such as a polymer or silica glass.
In the above structure, the optical mirror layer consists of a dielectric multilayer mirror with an optical reflectance of equal to or more than 95% for 1.5 xcexcm wavelength light, and the glass substrate has a finishing surface precision of equal to or less than xcex/10, where xcex is a wavelength of light that traverses the tunable wavelength-selective filter.
Further, in the above structure, the liquid crystal droplets consist of nematic liquid crystal droplets, and the polymer contains a plasticizer.
The wavelength-selective filter can be arranged in such a manner that it comprises on its second side one of a prism mirror and a plane mirror, wherein one of a light beam and a light beam array, which is incident on its first side, passes through it, reflects off one of the prism mirror and the plane mirror, and passes through it, and is emitted from its first side.
A fabrication method of a tunable wavelength-selective filter of a Fabry-Perot etalon type according to the present invention comprises the steps of: forming on a transparent substrate of equal to or less than 250 micrometers thick a first layer composed of a transparent electrode layer and an optical mirror layer; forming a second layer by uniformly applying on the first layer a fine liquid-crystal-droplet-dispersed polymer layer in which liquid crystal droplets 150 nanometers or less in diameter are dispersed in a matrix, and by polymerizing the fine liquid-crystal-droplet-dispersed polymer layer; forming on the fine liquid-crystal-droplet-dispersed polymer layer a third layer composed of a transparent electrode layer and an optical mirror layer; and dicing a filter fabricated through the previous steps into 5 millimeter or greater squares.
The fabrication method above can be further provided with the steps of adding a plasticizer to a mixed liquid of a liquid crystal material and a thermo polymerization or photo polymerization prepolymer; carrying out spinning; and carrying out heating or light irradiation.
According to the present invention, a polarization-independent tunable wavelength-selective filter with a high response rate and high speed tuning capability is implemented using the material in which the liquid crystal droplets are dispersed into the light transmissive characteristic medium.
According to the present invention, a tunable wavelength-selective filter which is cheaper in cost, higher in density and smaller in size than the conventional tunable wavelength-selective filters is implemented by combining with the arrayed optical fibers.
According to the present invention, stacking the tunable wavelength-selective filters in multiple layers or employing the planar mirror or the prism mirror can improve the spectrum characteristics, and extend the wavelength tunable range to all the wavelengths.
According to the present invention, employing the planar mirror makes it possible to simply construct the optical multiplexer or demultiplexer.
According to the present invention, a high speed LAN wavelength-division multiplexing transmission system can be constructed.
Finally, the tunable wavelength-selective filter in accordance with the present invention can greatly increase the extinction ratio of the Fabry-Perot etalon filter because it is possible for the Fabry-Perot etalon filters that use the fine liquidcrystal-droplet-dispersed polymer as their cavity to be stacked in multiple layers.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.